Hydrothermal synthesis and annealing-induced formation of exchange-coupled hexaferrite–nickel ferrite composite nanoplatelets

Ni-substituted barium hexaferrite nanoplatelets were synthesized by a hydrothermal method at 250 °C and their structural and magnetic evolution was investigated upon subsequent annealing. At low Ni contents (x ≲ 0.08 in BaFe12-xNixO19), Ni2+ was incorporated into the hexaferrite lattice, leading to a reduction in nanoplatelet size and a pronounced decrease in magnetization and coercivity. Aberration-corrected scanning transmission electron microscopy revealed incomplete occupation of the trigonal Fe(2b) sites in the as-synthesized nanoplatelets, accounting for the degraded magnetic properties. Upon annealing, the M-type hexaferrite structure was restored and the magnetic properties recovered.At higher Ni contents, exceeding the effective solubility limit, spinel nickel ferrite formed as a secondary phase and grew epitaxially on the basal surfaces of the hexaferrite nanoplatelets. This resulted in composite nanoplatelets composed of a thin hexaferrite lamella confined between two spinel ferrite layers. Annealing at 800 oC preserved this composite architecture and led to single-phase-like hysteresis loops, indicating rigid exchange coupling between the hard-magnetic hexaferrite and soft-magnetic spinel ferrite. Higher-temperature annealing (1000 oC) induced particle growth and produced irregular spinel–hexaferrite intergrowths that persisted even after prolonged annealing.The results demonstrate that Ni substitution and thermal treatment provide an effective route for controlling phase composition, intergrowth structure, and magnetic properties in hexaferrite-based nanocomposites, offering a tuneable platform for the design of exchange-coupled ferrite materials.